COUNT ME IN. Each starling in this flock adjusts its trajectory to those of its six or seven neighbors, no matter how close or far they are.STARFLAG Project/INFM-CNR

On winter evenings in some southern European towns, tens of thousands of starlings congregate over their roosts. Above the ruins of Rome's ancient Baths of Diocletian, huge black clouds of starlings assemble and continually morph into new shapes, possibly to signal their position to buddies who are still navigating their way home.

Scientists have proposed several explanations for how bird flocks, fish schools, and other large groups of animals coordinate their acrobatics, especially when they have to quickly change course to avoid predators, says Andrea Cavagna, a physicist at Italy's National Research Council (CNR) in Rome. The assumption has been that individuals match their trajectories to those of all animals within a given distance.

But precise observations were limited largely to movies of, say, fish schools, which scientists used to manually track the positions of single individuals, frame by frame. Scientists could study groups of a few dozen members at most.

Cavagna and his collaborators used computers to track the motions of single birds in flocks of up to 4,000 starlings flying over Rome. The team set up three pairs of high-speed digital cameras on a museum's rooftop, and developed software that matched the cameras' different views to reconstruct each bird's 3-D coordinates.

The results, which appear in the Jan. 29 Proceedings of the National Academy of Sciences, suggest that starlings count and then follow the six or seven neighbors closest to them, rather than all birds within a certain distance. It's like a computer network, Cavagna says, in which "what matters is not physical distance [between two nodes], but how many nodes there are in between." The team's computer simulations showed that such behavior most effectively keeps the flock together, even when the birds' average spacing changes.

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